If you're caught in a situation in which your rabbit is exposed to sudden
extreme heat (your air conditioner breaks or you're traveling), get some ice.
Buy large bags while on the road; at home freeze water in heavy-duty plastic
containers.

At home you can place frozen 2 liter bottles on the ground for your rabbit to
lay against.
Wet towels across the side of the cage combined with a natural or artificial
breeze, can produce evaporative air-cooling. Spraying bunny's ears with an
atomizer of water can cool him down, and so can a wading pool, if your bunny is
accustomed to, and not stressed by, such water sports.